I do not know for sure but I think it has more to do with the chamber of the rifle then the bullet it self . There still has to be something else though .

Just copied this from another site .I found it on the internet so it must be true

There is some overlap in the rounds, Im not sure of excact numbers but basically the 5.56 maximum overall length is longer then the max overall length for .223. So if you have a rifle chambered on the short side of .223 specs some 5.56 ammo wont work or be way over pressured when fired in the shorter chamber. If you are buying an Autoloader for volume shooting you should get a 5.56 chamber so you wont have problems with military surplus ammo

There are some 223 Remington rifles in which you wouldn't want to shoot 5.56x45 cartridges, some in which you wouldn't think twice, and some that you'd want to check before trying.
CIP standards (Europe) require pressure testing be done with the transducer at the case neck, where SAAMI cartridges typically measure with a transducer at the case head. You really can't translate directly from one to the other, but it's possible, perhaps even likely, that NATO loads will be at a higher pressure than is permissible in 223 cartridges, though not by a huge margin.
Also, it's been a long time since the standard military loads included a 55 grain bullet, and leades in military barrels tend to be pretty generous, giving up a little potential accuracy with those bullets to allow room to chamber the longer, heavier bullets common in military loads. Some civilian 223 rifles may not have the freebore to allow those bullets to be chambered safely, and if you're jamming the meplat of the bullet solidly into the lands, you're going to raise pressures.
If you have a strong bolt action like the Remington 700 action, it'll take a good deal of overpressure, though it isn't smart to get too smug about it. Additionally, if you want to be obsessive, you can measure the leade in your rifle to make sure there's a little room for the bullet to jump before it hits the lands. There are several ways to measure, some rather tedious, and probably the simplest/safest using a little gizmo made by Stoney Point.

__________________As of this date 8-18-14 at 6:42am I became a proud grandfather I guess I'm officially old
I hear Glock just came out with a new pistol . I hear it's a tribute to the senate and the house of representatives . It's called the congressman . It hardly ever works and can't be fired .

A .223 chamber has a shorter leade, or free bore, than a 5.56 chamber. It's theoretically possible for a heavy (longer) bullet in a 5.56 round to be jammed in to the start of the rifling in a .223 chamber. This could theoretically cause a over pressure situation. The bullet needs to have a short free space for it's initial movement.

I use "theoretically" because I don't see any reports of this happening, and a lot of peeps use 5.56 in .223 chambers.

Commercial .223 brass can be thinner than 5.56 NATO brass. The NATO brass may have a smaller volume than .223 brass.

Commercial .223 brass can be thinner than 5.56 NATO brass. The NATO brass may have a smaller volume than .223 brass.

Since 1968 i've been weighing the cases for my accuracy loads. Some British made 5.56mm military brass is very thick. US military 5.56mm brass is not heavier/thicker than .223 made commercial brass. The heaviest US made cases are Federal Gold Medal. The heaviest cases of all are those made by Lapua.

Since 1968 i have fired at least a half million rounds of US military M193 5.56mm ammunition in the .223 chambers of numerous guns with no ill effects. Some of my guns have been re-barreled five or more times. i would not attempt to fire a US military 5.56mm round in a .223 match chamber.

i'm not suggesting that you fire 5.56mm ammunition in the chamber of your .223 rifle.

supposedly it's mostly the pressure of the rounds with a very slight chamber difference but there are strong arguments that interchangeability is a myth and that it is not. I just either buy rifles chambered in 5.56 or 223 wylde and don't worry about it.

__________________
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
I never said half the crap people said I did-Albert Einstein
You can't believe everything you read on the internet-Benjamin Franklin
Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.

The big problem is shooting 5.56 in a .223 chamber and getting it hot. (Like at a carbine training class)

The tighter .223 chamber "can" cause the primers to "pop" out of the case of a 5.56 cartridge and fall into the trigger mechanism. If that gets into the wrong place it can deadline the rifle until you can get that primer out, which might entail taking the trigger group out of the rifle.

There are several examples of this in Pat Roger's and others AAR for classes. They would usually "fix" them with a 5.56 chamber reamer to open the chamber up some. Ned Christiansen makes a reamer for this.

I have never had or read about an issue with anything blowing up or other damage, but having a primer tie up a gun isn't something that you would want to happen in a defensive situation.

When something is 5.56 mm x 45 mm it meets specifications demanded by NATO. For example, the brass is a touch thicker, primers are harder, primers are crimped into the primer pocket (a pain for reloaders) and there is a laquer seal around the primer to prevent water from getting into the case. Also, depending on which version you obtain, it's either going to be 55 g or 62 g bullets.

Manufacturers of civilian (.223 Remington) ammunition don't have to meet these standards. They have others.

I believe that automatic weapons and semi automatic weapons are going to have slightly larger chambers than those that aren't. That extra bit of play makes them more reliable. I think that a 5.56 chamber is slightly larger than one that is .223. You should be able to fire .223 out of the 5.56 mm rifle, but could have a problem the other way around.

I was surprised when I loaded winchester brass in place of military brass. With military, I used 25.9 grains of 748, and with winchester it took 26.4 grains to get good results. Brass thickness made a significant difference.

I have not started reloading yet . I don,t even have one peace of reloading equipment but I do save all my brass for when I do . How can you tell them apart ( military brass vs commercial brass ) once they have been fired and all sitting in a bucket together . I my start a new thread asking this question in more detail

__________________As of this date 8-18-14 at 6:42am I became a proud grandfather I guess I'm officially old
I hear Glock just came out with a new pistol . I hear it's a tribute to the senate and the house of representatives . It's called the congressman . It hardly ever works and can't be fired .

I was surprised when I loaded winchester brass in place of military brass. With military, I used 25.9 grains of 748, and with winchester it took 26.4 grains to get good results. Brass thickness made a significant difference.

What was the headstamp on that military 5.56mm brass?

i've weighed US made 5.56mm and .223 brass cartridge cases since 1968 for my accuracy loads. i've weighed thousands of cases and never found US military 5.56mm cases to be thicker/heavier than US commercial .223 cases.

How can you tell them apart ( military brass vs commercial brass ) once they have been fired and all sitting in a bucket together

I don't know that there is a hard and fast set of rules.

I'd start by looking on the bottom of the case.

If you see a colored ring around the primer, it's probably military. Sometimes, the stamp will say .223 or 5.56. Sometimes you can tell by the manufacturer - that is two or three letters on the head stamp. LC is Lake City - that's probably military brass. I have some that are AD which was military made in the Middle East.

I'm sure there are folks who know more about this than I do. Your notion of starting a new thread on it is a good idea.

On 02/14/11 an officer was attending an M4/MP-5 class and during a shooting session, the .223 rifle (Stag Arms) experienced a catastrophic failure resulting in an upper receiver cracking and the bolt being pushed into the bolt assembley.

During a SWAT training day on 02/23/11, three more M4 style rifles experienced a catastrophic failure at the same time. All three rifles experienced the bolts being pushed into the bolt assembly, the upper receivers were cracked and / or broken apart, and the lower receivers were bowed outward. The ammunition we used in all the rifles were Winchester RA223R2 Ranger 64 gr. Power Point.

Supposedly 223 ammo is safe in either 223 or 5.56 chambers. The alleged problems are when 5.56 ammo is fired in 223 chambers.

All the examples above were from defective 223 ammo fired in 5.56 chambers. That ammo would have caused problems in any gun.

The problem is greatly overstated. There are minor differences between the 2 rounds and in chamber specs, but if you check out any other commercial chambering you will find differences just as great. Same with chambers in different guns.

Lots of talk about different chamber pressures. My loading manuals show anywhere between 27,000 psi up to 60,000 psi as a max charge in 30-06 depending on the load. I've found a few factory 30-06 loads that simply would not fit in the chambers of certain rifles if you happen to get a rifle with a chamber cut on the minimum side and ammo a little on he large side. We could easily have 2-3 different designations for 30-06, but we don't.

i've weighed thousands of cases and never found US military 5.56mm cases to be thicker/heavier than US commercial .223 cases.

This has been my experience also.

A few things should be kept in mind when over thinking this issue. One is that you have to trim the cases before weighing them so there is some uniformity to begin with. Another is that the difference between chambers is very likely talking about bolt action rifles in 223 vs semi autos in 223/5.56.

A bit about chambers, don't know if this is on topic or not but file it in the "for what its worth department"

Lot of talk about 223 chambers being tight vs the 5.56 chamber. Don't know for sure, I can only report on my chambers.

I have a Mann Accuracy Device (used by the military for testing ammo) I obtained from the CMP. Remington Action and Kart Barrel.

It's a TIGHT 5.56 chamber. If it isn't clean, it wont accept a 223 "go" gage.

About 1/3 of the pieces of new Winchester 223 brash wont chamber in the Mann device. It chambers in my two ARs and my Rem 700 Bolt gun.

I have to resize the new brass with a SB sizing die to use it in the Mann Device.

If I fire any ammo in my two ARs the brass wont chamber in the Mann Device. However if I fire ammo in my Remington 700 Varmint, I can get it into the Mann Device.

This Remington BDL Varmint is the rifle I carried as a LE Counter-sniper rifle. Got it in 1978. The ammo I carried and used in that rifle was M-193 5.56.

I shot it quite a bit wanting to keep in practice and constantly confirming zeros.

I had access to a lot of M193s back then and that's about all I shot. But the chamber is still, after nearly 35 years, the Remington Chamber is still tight enough to get the spent brass in my super tight Mann Chamber.

One of my ARs is a SP1, I've had it a while and have shot thousands of M193 out of it.

My other AR is a White Oak Service rifle. I got it later in life when I had to furnish my own ammo. Can't tell you much about it and M193, only fired a few rounds I found laying around.

This doesn't prove anything to anyone but me. I don't care what I shoot in my 223s, be it 223 or 5.56, in my rifles.

Do what makes you feel better.

We all have different opinions, That's good, if that wasn't the case we wouldn't have anything to talk about.

I doubt I get any more 223s, and I highly doubt I'll go back into the NG where I can have access to 5.56.

This email link is to reach site administrators for assistance, if you cannot access TFL via other means. If you are a TFL member and can access TFL, please do not use this link; instead, use the forums (like Questions, Suggestions, and Tech Support) or PM an appropriate mod or admin.

If you are experiencing difficulties posting in the Buy/Sell/Trade subforums of TFL, please read the "sticky" announcement threads at the top of the applicable subforum. If you still feel you are qualified to post in those subforums, please contact "Shane Tuttle" (the mod for that portion of TFL) via Private Message for assistance.

This email contact address is not an "Ask the Firearms Expert" service. Such emails will be ignored. If you have a firearm related question, please register and post it on the forums.